Substantial effort and expense have been devoted to the development of ink jet printers, especially during the past couple of decades. As is known, ink jet printing has the inherent advantage of being a plain paper compatible, direct marking technology, but the printers which have been developed to capitalize on that advantage have had limited commercial success. Although the reasons for the disappointing commercial performance of these printers are not completely understood, it is apparent that the persistent problems which have impeded the development of low cost, reliable print heads for them have been a contributing factor. Print heads have been provided for low speed ink jet printers, but they have not been fully satisfactory from a cost or a reliability point of view. Moreover, higher speed ink jet printing has not been practical due to the performance limitations of the available print heads.
"Continuous stream" and "drop on demand" print heads have been developed for ink jet printers. There are functional and structural differences which distinguish those two basic print head types from one another, but print heads of both types customarily include nozzles which have small ejection orifices for defining the size of the liquid ink droplets emitted thereby. They, therefore, suffer from many of the same drawbacks, including unscheduled maintenance requirements because of clogged nozzles and a fundamental cost barrier due to the expense of manufacturing the nozzles.
Others have proposed nozzleless print heads for ink jet printing. For example, Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 24, 1981 on a "Liquid Drop Emitter," pertains to acoustic print heads for such printers. This patent is especially noteworthy because one of its embodiments relates to a print head in which a hemispherically shaped piezoelectric transducer is submerged in a reservior or pool of liquid ink for launching acoustic energy into the reservior and for bringing that energy to focus at or near the surface of the reservior, so that individual droplets of ink are propelled therefrom. As will be seen, the patent also proposes an alternative embodiment which utilizes a planar piezoelectric crystal for generating the acoustic energy, a conical or wedged shaped horn for bringing the acoustic energy to focus, and a moving belt or web for transporting the ink into position to be propelled by the focused acoustic energy. However, the additional complexity of this alternative proposal is contrary to the principal purpose of the present invention.
A substantial body of prior art is available on the subject of acoustic liquid droplet ejectors in general. Some of the earliest work in the field related to fog generators. See Wood, R. W. and Loomis, A. L., "The Physical and Biological Effects of High Frequency Sound-Waves of Great Intensity," Phil. Mag., Ser. 7, Vol. 4, No. 22, September 1927, pp. 417-436 and Sollnar, K., "The Mechanism of the Formation of Fogs by Ultrasonic Waves," Trans. Faraday Soc., Vol. 32, 1936, pp. 1532-1536. Now, however, the physics of such ejectors are sufficiently well understood to configure them for ink jet printing and other applications where it is necessary to control both the timing of the droplet ejection and the size of the droplets that are ejected. Indeed, an inexpensive, reliable, readily manufacturable liquid droplet ejector providing such control is clearly needed for nozzleless ink jet printing and the like.